Title: REGULATION OF CERAMIDE SYNTHASE BY PROTEIN-PROTEIN INTERACTION Abstract: Ceramides form the backbone of all sphingolipids, and ceramide synthases (CerS) are critical enzymes for de novo production of ceramides. In spite of the key role of CerS in ceramide generation, there is a serious deficiency in understanding how these enzymes are regulated. The long-term goal of this project is to uncover and understand the fundamental molecular mechanisms of how CerS enzymes are regulated. Using a proteomics approach, we discovered that the small heat shock protein Hsp27 interacts specifically with CerS1. Based on our preliminary data we generated the novel hypothesis that Hsp27 is a negative regulator of CerS1 activity via direct interaction that can be modulated by p38-MK2 MAPK mediated phosphorylation of Hsp27, and that down-regulation of Hsp27 induces CerS1/C18:0-ceramide mediated cellular responses. To test this hypothesis, we propose the following Specific Aims: Aim 1: Define the biochemical significance of Hsp27 mediated CerS1 regulation in cells with respect to sphingolipid metabolism and signal transduction. Aim 2: Define the biological significance of Hsp27 mediated CerS1 regulation. Aim 3: Determine the mechanism of Hsp27- CerS1 protein-protein interaction. Overall, these studies will establish Hsp27 as an endogenous modulator of CerS1 and uncover a novel mechanism of how Hsp27 regulates CerS1 and CerS1/C18:0-ceramide governed mitophagy and cancer cell death. The knowledge generated from this study will help design mechanism-based novel therapies against cancer and other pathologies in which C18:0-ceramide is the key mediator by identifying new methods to modulate CerS1 activity.